Today's thermoset material systems used in commercial applications have very limited thermal resistance, and are manufactured on the basis of epoxy resins. However, this limited thermal stability is too low for many applications, particularly for environments in which increased thermal and mechanical loads occur. On the other hand, other commercially available materials for high-temperature applications, such as BMI or polyimide resins, are either toxic or extremely expensive and therefore not suitable for commercial use. Cyanate esters have an extremely high glass transition temperature of up to 400° C. This is due to the high crosslinking density of the thermoset network which however also means that the material is very brittle. Because of its brittleness until now this type of thermoset system has not found any commercial use in aviation. Moreover, most cyanate ester systems require a curing temperature higher than 200° C., which further affects the financial viability of said systems. Additionally, most of the curing agents used are harmful to health.
The materials should also exhibit good flame retardant properties. The flame retardants used should also not be associated with any toxic effects, and should not impair the physical properties of the materials, such as the glass transition temperature and mechanical properties. The flame retardants should also have the lowest possible migration tendency, and small quantities of the flame retardant should be sufficient to achieve the desired flame resistance. Many cyanate ester (CE)/epoxy combinations are already known.
For example, U.S. Pat. No. 5,494,981; US 2012/0309923; JP2009013205 (A); and C.-S. Wang, M. C. Lee: Synthesis, characterisation and properties of multifunctional naphthalene containing epoxy resins cured with cyanate ester, Journal of applied polymer science 73 (1999) 1611-1622, describe CE/epoxy mixtures that are produced with the aid of various curing agents, and catalysts respectively. These make use primarily of nonylphenol/transition metal catalysts as well as Brønstedt acids and amines. Transition metal catalysts are usually not soluble in the resin matrix, so phenols, such as nonylphenol are used as co-catalysts. Not only that there are health concerns surrounding the transition metal catalysts used, nonylphenol has been cited as a “substance of very high concern” (SVHC) since December 2012 by REACH [European Chemical Agency, Support document for identification of 4-Nonylphenol branched and linear, 13 Dec. 2012]. Additionally a significant disadvantage of Brønstedt-acids is that most of them are prepared on an aqueous basis. Since cyanate esters tend to form carbamates in the presence of water, which degrade into CO2 and amines at elevated temperatures, thereby impairing the performance of the materials, these catalysts should not be used.
Moreover, JP 08-176274 (A); JP 2010-059363; C.-S. Wang, M. C. Lee: Synthesis, characterisation and properties of multifunctional naphthalene-containing epoxy resins cured with cyanate ester, Journal of applied polymer science 73 (1999) 1611-1622, for example, describe materials prepared from cyanate esters and naphthalene-based epoxies. The naphthalene-based epoxies used in JP 08-176274 (A) are low-molecular and difunctional, which means that a naphthalene molecule has at most two glycidyl ether functionalities.
Because the naphthalene has such a low functionality, its crosslinking density is also low compared with more highly functionalised epoxies. In turn, low crosslinking density is also associated with lower thermal stability (Tg) and higher water absorption, but also reduced brittleness.
The epoxies used in JP 2010-059363 for preparing CE/epoxy materials are prepolymers based on polyphenylene with a naphthalene-functionality in the polymer backbone. These prepolymers only have one glycidyl ether functionality per repeating unit, which again results in a low crosslinking density. Moreover, unlike a backbone consisting only of naphthalene, the introduction of the polyphenylene ether group has the effect of increasing the molecular mobility of the polymer backbone. This might improve the mechanical properties of the material, but it would also lead to a decrease of the thermal properties (Tg). In Wang et. al., low-molecular, tetrafunctional naphthalene-based epoxies are used in CE/epoxy mixtures.
The advantage of these tetraglycidyl ether naphthalenes is that the resulting thermoset network exhibits enormously high crosslinking density and the associated good thermal as well as hygrothermal properties. In this way, it is possible to avoid lowering the Tg too far by mixing the epoxy into the cyanate ester. The curing process in this publication was carried out using nonylphenol/copper(II)acetyl acetonate, which is to be avoided at all costs for the reasons given above. The flame resistance of these systems can also be improved further.
Flame retardants can be added to polymers generally as simple additives (additive flame protection), or they may be copolymerised into the polymer matrix using specially functionalised monomers. However, if specially functionalised monomers are used, other properties of the resin matrix may be affected negatively. In the case of additive flame protection, the flame retardants used may tend to migrate more readily, since flame retardants used are not bonded to the polymer matrix.
Since cyanate esters already possess a high flame resistance because of their high aromatic content, the known literature contains only few examples in which cyanate esters have been chemically modified with flame retardants.
However, the resin matrix was chemically modified with the flame retardants before the curing process for this purpose. Such concepts are described for example in C. H. Lin, Polymer 2004, 45, 7911-7926; T.-H. Ho, H.-J. Hwang, J.-Y. Shieh, M.-C. Chung, Reactive and Functional Polymers 2009, 69, 176-182 and C. H. Lin, K. Z. Yang, T. S. Leu, C. H. Lin, J. W. Sie, Journal of Polymer Science Part A: Polymer Chemistry 2006, 44, 3487-3502.
Even if the physical and thermal properties of the polymer systems are not compromised thereby, further process steps are still necessary in order to modify the respective components. Furthermore, depending on the requirements profile the polymer must fulfil, it may also be necessary to use starter materials that have been modified differently, which renders their production more complicated and thus also more expensive.
Therefore, it would be desirable to provide a polymerisable thermoset composition that is resistant to high temperatures and has good flame retardant qualities. It should also be curable at moderate temperatures and should not contain any water-based compounds such as Brønstedt acids or curing agents or catalysts that are harmful to health, such as transition metal catalysts, and it should be modifiable in terms of impact resistance. It is further desirable to produce a polymerisable thermoset composition that is suitable for manufacturing lightweight construction components such as carbon fibre composites (CFRP).
Given the above, an embodiment provides a highly flame retardant, polymerisable thermoset composition, which additionally is resistant to high temperatures in the cured state, and particularly has increased thermal resistance, compared to pure epoxy resins. A further characteristic of the present embodiment is that in the cured state of the polymerisable thermoset composition, the flame retardant contained therein exhibits a low tendency to migrate.
Another characteristic of the present embodiment is that the polymerisable thermoset composition should not require a resin matrix that has been modified with flame protection agents before the curing process.
In accordance with certain embodiments, in the cured state the polymerisable thermoset composition has a high glass transition temperature, particularly a glass transition temperature, that is higher than those of the previously known CE/epoxy combinations. Still another characteristic of the present embodiment is that the polymerisable thermoset composition should have high impact resistance in the current state, particularly an impact resistance that is improved compared with pure CE/epoxy combinations. Another characteristic of the present embodiment is that the polymerisable thermoset composition should be curable at moderate temperatures, and in particular should have a lower curing temperature than pure cyanate esters. A further characteristic of the present embodiment is that the polymerisable thermoset composition should exhibit better resistance to hydrolysis than pure cyanate esters. Still another characteristic of the present embodiment is that the polymerisable thermoset composition should not contain any curing agents and/or catalysts that are injurious to health.
These and other features and characteristics are provided by the subject-matter defined in the claims. Advantageous embodiments constitute the subject matter of dependent claims.